That vast acreage - greater than the size of Denmark - is expected to rise over the remainder of the year, but it makes 2015 already the first year in at least five and a half decades to exceed 10 million acres burned, according to NIFC data.

A home burns as the Butte Fire rages through Mountain Ranch, California Photo: REUTERS/Noah Berger

That damage was done by 51,110 fires. By contrast, the last time US wildfires burned more than 9 million acres by October 8, in 2006, the number of fires totaled 84,578.

But NIFC data revealed a more worrying long-term trend: Since at least 1960 and up until the early 1980s, the US saw more than 100,000 individual wildfires each year, which, despite their numbers, never burned more than 5.5 million acres.

Between 2000 and 2014, however, the number of fires exceeded 90,000 only twice, but in nine of those years the total acreage burned exceeded 5.5 million.

A firefighter works to save a residence as the Butte fire burns in San Andreas, California Photo: REUTERS/Noah Berger

That rise in more-damaging "mega-fires" has been widely attributed to fire management practices, a growing number of homes in or near major forests and, especially, a trend toward hotter, drier seasons.

Those effects may be most obvious this year, as fires have torn through drought-parched California and other western states.

The more than 11 million acres burned by just 51,110 wildfires so far this year means each fire burned on average 220 acres - more than twice the average acreage per fire through this point in the year over the past decade, according to an AFP analysis of NIFC data.

NOTE: Since this article was published, the US government's National Interagency Fire Centre has issued a correction to state that as of October 9, 9,276,416 acres had been burned - not more than 11 million acres. For more information see the NIFC website. This year is still expected to be the worst year on record for wildfires, but by October 9 the figure had not passed the 9.8 million acres burned in 2006.